1. Field of the Invention
This invention relates to an improved method and apparatus for high velocity oxygen fuel (HVOF) thermal spray guns, and more particularly to a burner design to improve the combustion of oxygen and liquid fuel inside a spray gun combustion chamber.
2. Description of Related Art
The high velocity oxygen fuel (HVOF) process is used to produce dense hard coating structures on a variety of substrates and for a variety of applications, such as chrome replacement. HVOF coatings tend to have low residual tensile stress or in some cases have compressive stress, which enable thicker coatings to be applied than is typically possible with the other processes. The high kinetic energy of particles striking a substrate surface allows for the formation of high quality HVOF coatings even when the sprayed particles are not fully molten.
There are a number of HVOF guns which use different methods to achieve high velocity spraying. Use of these guns generally involves the use of either gas or liquid fuels. For liquid fuel guns, generally, a liquid fuel (primarily commercially available kerosene, although #1 diesel, #2 diesel, and similar light oils are also used) and oxygen are injected into a combustion chamber via a burner and the coating material is injected radially into the exit barrel downstream of the combustion chamber and downstream of a convergent/divergent nozzle to velocities in excess of 700 m/sec.
One of the limitations with the HVOF process using existing burner technology is excess carbon in the combustion process due to inefficient combustion. The presence of fuel rich mixture areas causes carbon particles to form that will not completely burn and can buildup on the combustion chamber walls or be ejected from the torch and become lodged in the coating.
Conventional HVOF gun designs have utilized coaxial mixing tubes to achieve liquid fuel atomization and subsequent combustion. Some other HVOF designs utilize the injection of a fuel gas along with the liquid fuel to assist in the atomization and flame anchoring in the combustion chamber. However, there remains a need in the art for a high velocity oxygen fuel spray system with improved efficiency, reduced carbon deposits, and smaller-sized equipment.